I've heard that 17" wheels might not fit over Speed 6 brakes, so, its probably a bad idea. But, I just thought I'd ask if anybody had any experience or opinions. Thanks.

Not a problem. I've got 17" wheels on mine. See post:
http://forum.mazda6club.com/index.php?showtopic=86533

You do need to pay attention to offset, however, like any other rim choice.

Thanks for the link. I agree with your reasoning. Now I wish I hadn't just bought my tires. I got F1 all seasons like you, but I got them in 18's. So, I could either just keep the wheels I got until the next time I need new tires, or, just get some 18 wheels. Hmmm...